Sending the User to Another App

One of Android's most important features is an app's ability to send the user to another app
based on an "action" it would like to perform. For example, if
your app has the address of a business that you'd like to show on a map, you don't have to build
an activity in your app that shows a map. Instead, you can create a request to view the address
using an Intent. The Android system then starts an app that's able to show
the address on a map.

As explained in the first class, Building
Your First App, you must use intents to navigate between activities in your own app. You
generally do so with an explicit intent, which defines the exact class name of the
component you want to start. However, when you want to have a separate app perform an action, such
as "view a map," you must use an implicit intent.

This lesson shows you how to create an implicit intent for a particular action, and how to use it
to start an activity that performs the action in another app. Also see the video embedded here
to understand why it's important that you include runtime checks for your implicit intents.

Build an Implicit Intent

Implicit intents do not declare the class name of the component to start, but instead declare an
action to perform. The action specifies the thing you want to do, such as view,
edit, send, or get something. Intents often also include data associated
with the action, such as the address you want to view, or the email message you want to send.
Depending on the intent you want to create, the data might be a Uri,
one of several other data types, or the intent might not need data at all.

If your data is a Uri, there's a simple Intent() constructor you can use to define the action and
data.

For example, here's how to create an intent to initiate a phone call using the Uri data to specify the telephone number:

Java

Other kinds of implicit intents require "extra" data that provide different data types,
such as a string. You can add one or more pieces of extra data using the various putExtra() methods.

By default, the system determines the appropriate MIME type required by an intent based on the
Uri data that's included. If you don't include a Uri in the
intent, you should usually use setType() to specify the type
of data associated with the intent. Setting the MIME type further specifies which kinds of
activities should receive the intent.

Here are some more intents that add extra data to specify the desired action:

Note: This intent for a calendar event is supported only with API
level 14 and higher.

Note: It's important that you define your Intent to be as specific as possible. For example, if you want to display an image
using the ACTION_VIEW intent, you should specify a MIME type of
image/*. This prevents apps that can "view" other types of data (like a map app) from being
triggered by the intent.

Verify There is an App to Receive the Intent

Although the Android platform guarantees that certain intents will resolve to one of the
built-in apps (such as the Phone, Email, or Calendar app), you should always include a
verification step before invoking an intent.

Caution: If you invoke an intent and there is no app
available on the device that can handle the intent, your app will crash.

To verify there is an activity available that can respond to the intent, call queryIntentActivities() to get a list
of activities capable of handling your Intent. If the returned List is not empty, you can safely use the intent. For example:

If isIntentSafe is true, then at least one app will respond to
the intent. If it is false, then there aren't any apps to handle the intent.

Note: You should perform this check when your activity first
starts in case you need to disable the feature that uses the intent before the user attempts to use
it. If you know of a specific app that can handle the intent, you can also provide a link for the
user to download the app (see how to link to your product on Google
Play).

Start an Activity with the Intent

Figure 1. Example of the selection dialog that appears
when more than one app can handle an intent.

Once you have created your Intent and set the extra info, call startActivity() to send it to the system. If the system
identifies more than one activity that can handle the intent, it displays a dialog
(sometimes referred to as the "disambiguation dialog") for the user to
select which app to use, as shown in figure 1. If there is only one activity that handles the
intent, the system immediately starts it.

Kotlin

startActivity(intent)

Java

startActivity(intent);

Here's a complete example that shows how to create an intent to view a map, verify that an
app exists to handle the intent, then start it:

Show an App Chooser

Figure 2. A chooser dialog.

Notice that when you start an activity by passing your Intent to startActivity() and there is more than one app that responds to
the intent, the user can select which app to use by default (by selecting a checkbox at the bottom
of the dialog; see figure 1). This is nice when performing an action for which the user
generally wants to use the same app every time, such as when opening a web page (users
likely use just one web browser) or taking a photo (users likely prefer one camera).

However, if the action to be performed could be handled by multiple apps and the user might
prefer a different app each time—such as a "share" action, for which users might have several
apps through which they might share an item—you should explicitly show a chooser dialog
as shown in figure 2. The chooser dialog
forces the user to select which app to use for the action every time (the user cannot select a
default app for the action).